Could Israel have experienced its national spiritual birth at Jesus' first coming?
Yes, Israel could have experienced its national spiritual birth at Jesus' first coming. He presented Himself as the King of Israel and performed signs proving who He was. However, the nation rejected Him, so God's timetable places that birth at the end of the Tribulation, at Christ's Second Coming.
Les explains that Paul uses the analogy of a due date for a birth. Paul calls himself one born out of due time, like a preemie delivered prematurely, because he was saved 2,000 years before Israel's national salvation. Les teaches that the nation of Israel will be born all at once, in a day. They could have experienced that national spiritual birth at Christ's first coming, when He presented Himself as the King of Israel and performed signs to prove who He was. But they rejected it. So God's timetable places Israel's birth clear out to the end of the Tribulation, at the Second Coming of Christ. Then they will see their resurrected Lord, Messiah, and King coming in the clouds of glory and believe in a moment. Not all Israel, but the remnant, will yet be saved.